“Our own Universe may be the interior of a black hole existing in another universe.” In a remarkable paper about the nature of space and the origin of time, Nikodem Poplawski, a physicist at Indiana University, suggests that a small change to the theory of gravity implies that our universe inherited its arrow of time from the black hole in which it was born.

Poplawski says that the idea that black holes are the cosmic mothers of new universes is a natural consequence of a simple new assumption about the nature of spacetime. Poplawski points out that the standard derivation of general relativity takes no account of the intrinsic momentum of spin half particles. However there is another version of the theory, called the Einstein-Cartan-Kibble-Sciama theory of gravity, which does.

This theory predicts that particles with half integer spin should interact, generating a tiny repulsive force called torsion. In ordinary circumstances, torsion is too small to have any effect. But when densities become much higher than those in nuclear matter, it becomes significant. In particular, says Poplawski, torsion prevents the formation of singularities inside a black hole.

Astrophysicists have long known that our universe is so big that it could not have reached its current size given the rate of expansion we see now. Instead, they believe it grew by many orders of magnitude in a fraction of a second after the Big Bang, the period known as known as inflation.

Poplawski's approach immediately solves the inflation problem, saying that torsion caused this rapid inflation, which means the universe as we see it today can be explained by a single theory of gravity without any additional assumptions about inflation.

Another important corollary of Poplawski's approach is that it makes it possible for universes to be born inside the event horizons of certain kinds of black hole where torsion prevents the formation of a singularity but allows energy density to build up, which leads to the creation of particles on a massive scale via pair production followed by the expansion of the new universe. "Such an expansion is not visible for observers outside the black hole, for whom the horizon's formation and all subsequent processes occur after infinite time," says Poplawski. For this reason, he emphasizes, the new universe is a separate branch of space time and evolves accordingly.

Poplawski's theory also suggests an solution lto why time seems to flow in one direction but not in the other, even though the laws of physics are time symmetric.

Poplawski says the origin of the arrow of time comes from the asymmetry of the flow of matter into the black hole from the mother universe. "The arrow of cosmic time of a universe inside a black hole would then be fixed by the time-asymmetric collapse of matter through the event horizon," he says.. Translated, this means that our universe inherited its arrow of time from its source. "Daughter universes," he says, "may inherit other properties from their mothers," implying that it may be possible to detect these properties, providing an experimental falsifiable proof of his idea.